The sentence "Narwhals rub their tusks together for cleaning." is grammatically correct!
Here's why:
- Subject: "Narwhals" - the animals we are talking about.
- Verb: "rub" - what the narwhals are doing.
- Object: "their tusks" - what they are rubbing.
- Additional information: "together for cleaning" - explains why they are rubbing their tusks.
Everything is clear and makes sense!